This invention relates in general to semiconductor lasers and in particular, to a semiconductor injection laser whose active region is doped by ions such as rare earth ions to produce a single frequency semiconductor laser.
Single frequency lasers are of great practical importance in coherent optical telecommunication and other applications such as instruments. A number of techniques have been proposed for achieving single frequency lasers. One basic approach is to couple a laser which emits light within a bandwidth of frequencies with a filter in a feedback path in order to enhance light emission at a discrete resonant optical frequency. However, the fabrication of these distributed feedback lasers is complicated resulting in low yield.
One of the most elegant and simple techniques to achieve a single frequency laser is to dope a semiconductor junction with erbium ions. In this technique, erbium ions are introduced into the semiconductor through ion implantation, molecular beam epitaxy or liquid phase epitaxy. The erbium ions are excited when the host semiconductor is either optically or electrically excited/injected with electrons and holes. The exact nature of energy transfer between the erbium and the electrons/holes is unknown. However, when the erbium ions are excited by optically or electrically exciting/injecting the host semiconductor by electrons and holes, the erbium emits light at its atomic transition of approximately 1.54 microns. This atomic linewidth is potentially extremely narrow. Electroluminescence and photoluminescence of rare earth elements in compound semiconductor media such as indium phosphide and gallium arsenide are discussed by various publications by a number of authors. See, for example, the following:
1. H. Ennen et al., "Rare Earth Activated Luminescence in InP, GaP and GaAs" J. Crystal Growth, 64 (1983) 165-168. PA1 2. A. G. Dmitriev et al., "Electroluminescence of Ytterbium-doped Indium Phosphide," Soviet Phys. Semicond., 17(10) 1983, 1201. PA1 3. W. T. Tsang et al., "Observation of Enhanced Single Longitudinal Mode Operation in 1.5 um GaInAsP Erbium-doped Semiconductor Injection Lasers," Appl. Phys. Letters, 49 (25), 1986, 1686-1688. PA1 4. J. P. Van Der Ziel et al., "Single Longitudinal Mode Operation of Er-Doped 1.5 um InGaAsP Lasers," Appl. Phys. Letters, 50 (19) 1987, 1313-1315.
In the above-referenced article, Tsang and Logan matched the atomic transition of erbium with a semiconductor transition, with the atomic erbium transition energy slightly greater than the semiconductor (GaInAsP) transition. They reported single mode operation of the erbium-doped GaInAsP diode laser.
The technique used for incorporation of the rare earth ions into the semiconductor material is critical in developing a successful device. While liquid phase epitaxy and molecular beam epitaxy may be used, ion implantation offers better ion spatial distribution and doping control compared to liquid phase epitaxy or molecular beam epitaxy for the incorporation of these heavy ions. In both liquid phase epitaxy and molecular beam epitaxy, the erbium ions tend to cluster at heterojunction interfaces and cause inhomogeneities in the epitaxial layers (see reference 4 above).
In ion implantation, epitaxial layers are first grown and the rare earth ions then introduced through implantation. Therefore, the implanted ions will have minimal disturbance on the epitaxial quality. The ions are then activated by high temperature annealing. Since the rare earth elements are heavy, typically they are implanted only to shallow depths in the epitaxial layers. Hence they will remain close to the surface of the epitaxial layers. When excited, they will emit light at the linewidth of 1.54 microns. However, since they remain close to the surface of the epitaxial layers, it is difficult to confine the light emitted. It is therefore desirable to provide a semiconductor structure and system of manufacture in which such difficulties are overcome.